Free radical polymerization method having reduced premature termination, apparatus for performing the method, and product formed thereby

ABSTRACT

A method of polymerizing by a free radical polymerization mechanism, product formed thereby, and apparatus for performing this method, are disclosed. The composition to be polymerized by the free radical polymerization mechanism is irradiated by a substantially constant radiation, the radiation being substantially without pulsation. The use of the substantially constant radiation without pulsation reduces premature termination of the polymerization. The substantially constant radiation can be the output of a lamp powered by a constant current, direct current power supply.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Continuation application of prior application Ser.No. 09/983,208, filed Oct. 23, 2001, the contents of which areincorporated herein by reference in their entirety, which Ser. No.09/983,208 claims the benefit under 35 USC 119(e)(1) of provisionalapplication Ser. No. 60/300,816, filed Jun. 27, 2001.

BACKGROUND

The present invention is directed to a polymerization method, whereinthe polymerization occurs by a free radical polymerization mechanism, toapparatus used for this method, and to the polymerization productproduced by this method, and particularly wherein premature terminationof the polymerization is reduced. The present invention is applicable topolymerization of materials wherein the polymerization mechanism is afree radical polymerization, without limitation to the material beingpolymerized, and is applicable, for example, in the polymerization ofmaterials with C═C double bonds where the polymerization is by a freeradical polymerization mechanism, initiated by irradiation with, e.g.,light (such as ultraviolet light).

In a conventional photoinitiated, free radical polymerization process,using, for example, a microwave driven, electrodeless ultraviolet lamp,the microwaves are produced utilizing rectified alternating current, ora pulsed power supply. These microwaves produce pulsed ultraviolet lightfrom the electrodeless ultraviolet lamp, as the photoinitiating light.That is, the rectified alternating current or pulsed power supplyproduces a pulsed light output from the lamp for photoinitiation of thepolymerization. However, this conventional process forms a product withan undesirably low molecular weight, and has an undesirably low yield.

Thus, it is desired to form a polymerization product, produced by a freeradical polymerization mechanism, having increased yield and increasedmolecular weight, with a narrower distribution in molecular weight ofthe product (polymer) formed, while being formed at a relatively highspeed. That is, it is desired to improve the product produced by a freeradical polymerization mechanism initiated by radiation (e.g., light,such as ultraviolet light), both in yield (increased yield), and inmolecular weight of the product formed (increased molecular weight) anddecreased distribution (range) in the formed product.

SUMMARY

The present inventors have found that premature termination of the freeradical polymerization, causing disadvantageously low yield and productsof disadvantageously low molecular weight, is attributable to the pulsedoptical output providing photoinitiation of the free radicalpolymerization. Based on this finding, the present invention appliesconstant radiation (e.g., light), for example, constant light intensity,over time, during the polymerization process, to, e.g., initiate thepolymerization (to provide the photoinitiation) and sustain thepolymerization.

Moreover, according to the present invention, the constant light outputcan be achieved by use of a constant output (temporally), direct currentpower supply. For example, and not to be limiting, the constant output(e.g., constant current) direct current power supply can be used forgenerating microwaves for driving a microwave-powered lamp, whichproduces a constant(uniform) continuous light output, over time, forphotoinitiation, thereby reducing premature termination. According toaspects of the present invention, the constant (uniform) current, directcurrent power supply, for example, can be used to provide a constantcurrent, over time, applied to a magnetron to generate, e.g., themicrowaves driving the lamp, which microwaves produce a constant lightoutput from the lamp (electrodeless lamp) to cause polymerization toproceed. By providing a constant light output (constant intensity,having no ripple), premature termination is reduced and undesirableeffects arising due to premature termination are at least reduced.

As another example, a constant output (e.g., constant current) directcurrent power supply can be used to power an arc lamp to produce aconstant (uniform) continuous light output (constant intensity, havingno ripple), over time, for photoinitiation, thereby reducing prematuretermination.

Accordingly, an improved polymer product can be formed, having improvedproperties, and higher molecular weight and decreased distribution(range) of molecular weights, and the polymer product can be produced athigher speeds (e.g., when forming a sheet material, the cured productcan be formed at higher line speeds).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1( a) schematically shows a system for curing (polymerizing) apolymer according to a convention technique, using a microwave drivenultraviolet (UV) lamp to supply energy for initiating the polymerizationand a rectified alternating current or pulsed direct current powersupply in generating the microwaves, with FIG. 1( b) showing the currentoutput as a function of time for the power supply for generating themicrowaves for driving the microwave driven UV lamp, and with FIG. 1( c)showing the UV light output of the lamp as a function of time.

FIG. 2( a) schematically shows a system for curing (polymerizing) apolymer according to an illustrative embodiment of the presentinvention, using a microwave driven ultraviolet lamp to supply energyfor initiating the polymerization and a direct current power supply ingenerating the microwaves, with FIG. 2( b) showing the current output asa function of time for the power supply for generating the microwavesfor driving the microwave driven UV lamp, and FIG. 2( c) showing the UVlight output of the lamp as a function of time.

FIG. 3 is a circuit diagram of a direct current power supply which canbe used, according to an illustrative embodiment of the presentinvention, in generating the microwaves driving a microwave driven lampproducing constant light output to initiate polymerization.

DETAILED DESCRIPTION

While the invention will be described in connection with specific andpreferred embodiments, it will be understood that it is not intended tolimit the invention to those embodiments. To the contrary, it isintended to cover all alterations, modifications and equivalents as maybe included within the spirit and scope of the invention as defined bythe appended claims.

Throughout the present specification, where materials, methods andapparatus are described as including or comprising specific componentsor structure or specific processing steps, it is contemplated by theinventor that materials, methods and apparatus of the present inventionalso consist essentially of, or consist of, the recited components orstructure or recited processing steps.

The present invention, in illustrative aspects thereof, contemplates afree radical polymerization process, apparatus for performing theprocess and improved product formed. According to an illustrativeprocess within the present invention, the composition containing amaterial (e.g., monomer or oligomer) to be polymerized by free radicalpolymerization, is irradiated with substantially constant radiation,substantially without pulsation, to, e.g., initiate and sustain the freeradical polymerization, during the time period that the composition isbeing subjected to polymerization. This processing produces an improvedproduct, having improved properties, and produces the product at higherspeeds (higher throughput). Moreover, apparatus for producing thesubstantially constant radiation includes a radiation source (e.g., butnot to be limiting, a source of light, such as ultraviolet light)producing the substantially constant radiation, substantially withoutpulsation; and, e.g., a direct current power supply which outputs aconstant current used to generate the substantially constant radiationsubstantially without pulsation. Illustratively, the source of theradiation can be a microwave driven UV lamp, with the direct currentpower supply providing a constant current applied to a microwavegenerator to produce microwaves used to drive the UV lamp. Through useof the substantially constant radiation, substantially withoutpulsation, premature termination of the free radical polymerization canbe at least reduced.

Premature termination describes the situation where growingmacroradicals, in the formation of the polymer by the free radicalpolymerization mechanism, are terminated by small primary radicals fromphotolysis of the photoinitiator. This happens under pulsed irradiation,where the pulsing frequency is shorter than the time needed to reach“steady state” (steady state being that state where the rate of radicalformation is equal to the rate of termination (rate of radical-radicalcoupling)). Growing macroradicals (propagation phase) will have a lowerrate of termination, due to mobility restrictions caused by the veryfast increase in viscosity of the polymerizing medium and the size ofthe macroradicals. After the first pulse of light, most of the primaryradicals will promote propagation and, with the free radicalpolymerization, add into the double bond (for example, acrylate doublebond, where the monomeric material being polymerized is an acrylate).With further short pulses, the remaining unphotolyzed photoinitiatorwill now start to produce new small primary radicals, which have a muchhigher mobility than the growing macroradicals. So, in essence thesenewly generated small primary radicals from a subsequent photolysis ofthe photoinitiator (arising from pulsed UV light) will promote biradicaltermination by coupling with growing macroradicals in competition withinitiation of new chain growth, resulting in premature termination ofthe polymerization process.

By avoiding pulsed light, and using a constant light output, over time,without pulsation, according to the present invention, photolysis ofphotoinitiators which preferentially terminates polymerization isreduced. In addition, by using a constant current DC power supply topower the lamp providing the light (for example, to generate microwavesfor driving the lamp, where the lamp is a microwave-driven electrodelesslamp; or to power an arc lamp), a constant light output (e.g., constantultraviolet light output) can effectively be achieved, to reduce theabove-described premature termination and at least reduce undesirableeffects thereof.

While the present invention is described herein primarily in connectionwith free radical polymerization using photoinitiator activated byultraviolet light, the present invention is not limited thereto. Thatis, the present invention includes use of constant light output, withoutany substantial pulsation or ripple, of other types of light (e.g.,visible light, infrared light, etc.), and of other types of radiation,depending upon the energy (e.g., light energy) needed to activate thefree radical polymerization (for example, and not to be limiting, toactivate an initiator for initiating the free radical polymerization).

FIG. 1( a) shows the conventional technique for free radicalpolymerization of a composition which forms a work surface 7 (e.g., acoating on a substrate). This conventional technique uses a rectifiedalternating current or pulsed direct current power supply 1 forgenerating microwaves to drive microwave-driven UV lamp 3, to produceultraviolet light 11 to activate photoinitiators in the compositionforming work surface 7. FIG. 2( a) illustrates the present invention,using a DC power supply 5 for driving a microwave driven ultravioletlamp 13 to produce ultraviolet light 15 to activate the photoinitiatorsin the composition forming work surface 7, to initiate free radicalpolymerization. FIGS. 1( b) and 2(b) respectively show the currentproduced by the power supplies 1, 5 as a function of time; and shown inFIGS. 1( c) and 2(c) are the light output 11, 15 respectively producedaccording to the conventional technique and according to the presentinvention, and applied to the work surface 7, as a function of time. Thearrow designated by reference character 9 denotes the direction ofmovement of work surface 7 past the lamps 3, 13.

According to the conventional technique power supply 1 produces pulsedcurrent, shown in FIG. 1( b); and ultraviolet light 11 produced by lamp3 is pulsed, as seen in FIG. 1( c). This pulsed light causes prematuretermination of the free radical polymerization, as discussed previously.

As can be seen in FIGS. 2( b) and 2(c), according to the presentinvention a constant current is supplied from power supply 5, andproduces a constant light output 15 from ultraviolet lamp 13 over time,applied to the work surface; as discussed previously, this reducespremature termination so as to at least reduce undesirable effects andprovide increased yield and higher molecular weight products.

A DC power supply which can be utilized according to aspects of thepresent invention is shown in FIG. 3. This DC power supply isillustrative of a power supply which can be used in the presentinvention, and is not limiting of the present invention. This powersupply illustratively provides a high voltage output, having a constantcurrent over time, to, e.g., a magnetron, where the lamp is anelectrodeless lamp driven by microwaves. The magnetron generates themicrowaves for driving the lamp, which produces a light output which isconstant over time. The DC power supply can also be used to power arclamps.

More specifically, FIG. 3 shows a three-phase AC line voltage that isrectified by a three-phase bridge 20 after passing through line filtersand contacts. The three-phase bridge 20 may provide a DC voltage to an Hbridge 30. A pre-charge relay (and associated resistor) 25 and acapacitor 27 may be provided between the three-phase bridge 20 and the Hbridge 30. The pre-charge relay (and associated resistor) 25 may preventdamage caused by in-rush current that is used to charge the capacitor27. The bridge 30 may include four insulated gate bipolar transistors(IGBT) arranged as an H. In operation, two diagonal transistors of the Hbridge 30 may conduct simultaneously for one half of a switching cycleand then the other diagonal transistors of the H bridge 30 may conductfor the remaining one half of the switching cycle. This operation mayprovide alternating current to the high voltage transformer andrectifier (HVTR) assembly 40. The transistors may operate at a highfrequency (such as approximately 20 kHz) so that the size and the weightof the high voltage transformer can be reduced. The filtering componentsthat smooth out the current to the magnetron may also be reduced whenhigher operating frequencies are used.

A phase shifter/pulse width modulator 50 may perform the control of theIGBT transistors. The inputs to the phase shifter/pulse width modulator50 may be the magnetron current and voltage, which are measured on theHVTR assembly 40. The phase shifter/pulse width modulator 50 may adjustthe control signals to the transistors so that it regulates either thecurrent or the voltage at the output of the HVTR assembly 40. Thisprocess may be called feedback control.

The HVTR assembly 40 may include a multi-output transformer. Each outputmay be rectified with an individual rectifier and filter circuit. Thefilter circuit may include energy storage elements (such as inductorsand capacitors) to smooth out the high frequency pulses and supplyessentially DC current to the magnetron. The individual rectifieroutputs may be series connected to generate the high voltage required bythe magnetron.

An engine control circuit 60 may monitor and control the entire powersupply as required. The engine control circuit may include a programmedmicroprocessor with necessary input/output circuitry to monitor internalvoltages and currents.

The present invention is not limited to polymerization of any specifictype of monomer(e.g., acrylates), and can be applied to anypolymerization occurring by a free radical polymerization mechanism(that is, free radical systems). For example, any kind of free radicalpolymerizable C═C double bond, regardless of other structure in themolecule, will be more or less sensitive to premature termination, andsuch premature termination can be reduced by applying the presentinvention using a constant light output applied to the composition to bepolymerized.

In the following will be discussed various groups of materials to whichthe present invention can be applied. However, it is to be emphasizedthat the following are only examples of, e.g., commercially availableclasses of reactive systems sensitive towards premature termination,since they undergo free radical polymerization, and the presentinvention is not to be limited to these classes of reactive systems.

A first class of materials to which the present invention is applicableis those which form homopolymers by free radical polymerization, thatis, homopolymerization of the chain growth type. In this class, only onetype of double bond is used; the double bond, however, can be chemicallybonded to any kind of backbone, and any number of C═C double bonds canbe attached to any backbone. Typical examples are acrylates andmethacrylates. Styrene and styrene derivatives, N-vinyl amides, andvinyl esters are also typical examples of C═C structures which can bepolymerized by free radical polymerization and benefit from use of theconstant direct current power source providing constant light outputover time, according to the present invention.

A second class of materials to which the present invention is applicableis those which form copolymers by copolymerization of the chain growthtype. This uses different monomers or oligomers, discussed above inconnection with homopolymerization, together. Mixtures of any number ofdifferent monomers or oligomers, in any ratio and any type of C═C doublebond described above, can be used; depending on the reactivity ratio ofthe individual monomers/oligomers, the copolymer formed will have verydifferent properties. Typical examples are acrylate/methacrylate,acrylate/unsaturated polyester and acrylate/N-vinyl amide.

A third class of materials to which the present invention is applicableis those which form copolymers by alternating copolymerization. In thiscase, any acceptor (A) type monomer or mixtures thereof, in any numberand in any ratio, in a mixture (close to 1:1 on a molar ratio) with anydonor type (D) monomer or mixtures thereof in any number and in anyratio, will undergo alternating copolymerization. For this case, thereare an additional number of potential and commercially availablemonomers and oligomers containing C═C bonds with substituents previouslynot mentioned. These include A and D type monomers that will not undergohomopolymerization as discussed previously. Various acceptor and donortype monomers are described in connection with U.S. Pat. No. 5,446,073to Jonsson, et al., the contents of which are incorporated herein byreference in their entirety. Typical examples are fumarate/vinyl ether,maleate/vinyl ether and maleimide/vinyl ether.

As seen in the foregoing, many different types of polymerizationreactions, occurring through free radical polymerization, will benefitfrom the present invention. The foregoing classes are not the onlymaterials that can be used, and other classes and types of materials canbe utilized, as long as polymerization occurs through free radicalpolymerization.

The present invention is applicable to any polymerization techniquewhich occurs by free radical polymerization. Monomers and oligomers canbe polymerized according to the polymerization of the present invention.Monomers which can be polymerized according to the present inventioninclude molecules containing at least one C═C, that can be polymerizedby a free radical induced process. Such monomer can contain any number,in any ratio, of all kinds of C═C bonds, capable of undergoing a freeradical polymerization.

In the following are set forth examples of various aspects of thepresent invention, together with comparisons for showing the betterresults achieved according to these aspects of the present invention.These examples according to the present invention use a constant current(over time), direct current power supply, and provide a constant lightoutput applied to the composition to be polymerized; and are comparedwith examples using a variable power supply which provides irradiatinglight having a ripple (pulsed light). The compositions irradiated so asto undergo free radical polymerization were provided in a film thicknessof 15 or 25 microns, which was controlled by a Teflon spacer or awire-wound draw-down bar. The coated films were cured both in thepresence of air and in the absence of air. Each film passed by theirradiating lamp at the stated speeds (fpm: feet per minute) set forthin the following Table 1. The compositions contained Irgacure 184, whichis 1-hydroxycyclohexyl phenyl ketone, as a photoinitiator. Thecompositions contained the Irgacure 184 in amounts as set forth in thefollowing, and contained isobornyl acrylate, HDDA (hexanedioldiacrylate) or ethoxylated nonylphenol acrylate as the monomer whichpolymerizes by free radical polymerization. CHVE (cyclohexanedimethanoldivinylether) and EMI (N-ethyl maleimide) can also be used as themonomer in free radical polymerization of the photoinitiator system.

The examples in the following, corresponding to the present invention,utilized an “OMNI” DC power supply (see the power supply diagram in FIG.3) supplying a constant current to a microwave-driven ultraviolet lightlamp, which irradiated the composition with a constant ultraviolet lightoutput over time. The “VPS-3” power supply, for the comparativetechnique, provided a pulsed electric current which, when applied to themicrowave driven ultraviolet light lamp, provided an ultraviolet lightoutput that was pulsed (rippled).

From the following results, it can be seen that the double bond(C═C)conversion of formulations cured according to the present invention canbe about 7% higher than that achieved using a comparative technique. Ascan be seen in the following, advantageous results achieved according tothe present invention are particularly great at higher speeds ofmovement of the substrate having the coating film thereon, past theirradiating light.

TABLE 1 A. Homopolymerization - Isobornyl acrylate containing 0.25%Irgacure 184 25 fpm 50 fpm 100 fpm 150 fpm 200 fpm VPS-3 91% 89% 84% 69%51% 91% 90% 82% 67% 44% 92% 90% 84% 69% 44% 52% Average: 91% 90% 83% 68%48% OMNI 92% 91% 85% 73% 55% 92% 91% 85% 72% 55% 91% 91% 85% 73% 53% 44%Average: 92% 91% 85% 73% 52%

TABLE 2 B. Homopolymerization - Isobornyl acrylate containing 2%Irgacure 184 25 fpm 50 fpm 100 fpm 150 fpm VPS-3 96% 95% 91% 87% 96% 95%92% 88% 96% 96% 92% 86% Average: 96% 95% 92% 87% OMNI 96% 95% 92% 90%96% 95% 92% 90% 97% 95% 91% 90% Average: 96% 95% 92% 90%

TABLE 3 C. Homopolymerization - Hexanediol diacrylate (HDDA) containing0.125% Irgacure 184 25 fpm 50 fpm 100 fpm 150 fpm VPS-3 85% 80% 65% 42%87% 81% 62% 45% 86% 83% 60% 39% 41% Average: 86% 81% 62% 42% OMNI 86%84% 66% 46% 86% 83% 68% 49% 84% 66% 46% 53% Average: 86% 84% 67% 49%

TABLE 4 D. Homopolymerization - Ethoxylated nonylphenol acrylatecontaining 0.3% Irgacure 184 Difference of double bond conversion usingOMNI and VPS-3 at different cure speed 75 fpm 100 fpm 125 fpm 150 fpmVPS-3 78.1% 62.5% 57.4% 53.3% 77.6% 66.7% 53.9% 50.5%   80% 67.1% 58.6%47.6% 78.6% 68.2% 58.5% 45.9%   76% 63.9% 52.9% 49.9% Average: 78.1%65.7% 56.3% 49.4% OMNI 74.8% 63.8%   59% 50.3% 77.9% 72.2% 63.6% 50.1%81.2% 71.7% 62.2% 50.9% 85.9%   69% 62.5% 52.6% 78.4% 66.4% 57.3% 52.7%Average: 79.6% 68.6% 60.9% 51.3%

TABLE 5 E. Copolymerization - Isobornyl acrylate/1,6-HDDA (1:1 molarratio) containing 0.2% Irgacure 184 Difference of double bond Conversionusing OMNI and VPS-3 at different cure speeds 50 fpm 75 fpm 100 fpm 125fpm 150 fpm VPS-3 89.6% 83.9% 80.4% 70.2%   54% 89.9% 84.9% 73.6% 71.1%50.9% 88.9% 84.3% 79.1% 67.4% 50.4% 87.3% 86.3% 77.7%   58% 46.9% 91.4%86.8% 79.7% 61.2% 41.1% Average: 89.4% 85.2% 78.1% 65.6% 48.7% OMNI90.9% 83.8% 82.8%   71%   59% 90.4% 86.9% 83.3% 77.3% 54.6% 90.9% 87.8% 2.1% 72.6% 50.6% 85.7% 80.9% 76.5% 49.2% 89.5% 82.1%   64% 47.9%Average: 90.7% 86.7% 82.2% 72.3% 52.3%

TABLE 6 F. Alternating Copolymerization - Cyclohexanedimethanoldivinylether/N-ethyl maleimide (1:2 molar ratio) Difference of doublebond conversion using OMNI and VPS-3 at different cure speeds 75 fpm 100fpm 125 fpm VPS-3 86.3% 76.4% 73.5% (maleimide) 88.8% 79.9% 74.2% 70.7%74.4% 69.2% 80.5% 88.6% 77.1% 79.8%   63% Average: 81.2% 76.5% 73.5%VPS-3 61.0%   57% n/a (vinylether) 59.2%   57% 51.6%   56% 56.3% 67.7%55.5% 44.2% Average: 56.7% 56.4% OMNI   91% 79.2% 74.7% (maleimide)94.6% 82.9% 82.5% 78.2% 73.8% 82.8% 89.9%   94% 76.2% 82.7% 81.8%Average: 87.3% 82.3% 79.1% OMNI 64.5% 61.8% 57.5% (vinylether) 68.8%65.4% 59.4%   60% 53.6% 59.8% 65.1% 63.1% 58.9% 62.2% 60.4% Average:64.1% 60.9% 58.9%

In Tables 7 and 8 are shown a comparison of Gel PermeationChromotography (GPC) measurements made on samples cured using an “OMNI”power supply, according to the present invention, and, as a comparison,using the “VPS-3” power supply. The resulting molecular weight (MW)distribution clearly shows a much more narrow distribution for samplescured using the “OMNI” power supply. This is due to the fact that thefraction of short polymer chains is much smaller when the “OMNI” powersupply is used.

TABLE 7 G. GPC results of Isobornyl acrylate/Irgacure 184 (0.25%), curedwith Nitrogen Inerting at different curing speeds Retention time (min.)Mw Mw* d(peak width) VPS-3/HP6(10 fpm) 15.392 94177 106889 3.1280OMNI/HP6(10 fpm) 15.450 90079 103974 3.0639 VPS-3/HP6(200 fpm) n/a n/a230923 4.8724 OMNI/HP6(200 fpm) n/a n/a 197038 4.7655 Mw is an averagemolecular weight at retention time. Mw* is a whole average molecularweight distribution.

TABLE 8 H. GPC results of Isobornyl acrylate/Irgacure 184 (2%), curedunder air at different curing speeds Retention time (min.) Mw Mw* d(peakwidth) VPS-3/HP6(10 fpm) 15.945 62405 68089 2.2914 OMNI/HP6(10 fpm)15.990 60357 68408 2.1793 VPS-3/HP6(20 fpm) 16.186 52193 56010 2.2848OMNI/HP6(20 fpm) 16.239 50182 55842 2.2150 VPS-3/HP6(200 fpm) 17.24023889 50471 2.4988 OMNI/HP6(200 fpm) 17.237 23942 43293 2.2061 Mw is anaverage molecular weight at retention time. Mw* is a whole averagemolecular weight distribution.

Accordingly, by the present invention, utilizing constant outputradiation (for example, constant output light, such as ultravioletlight, but not limited thereto; other initiating light, such as infraredand visible light can be utilized), premature termination of thepolymerization can be reduced. Moreover, conversion of monomer to thepolymer can be improved, particularly at high rates of speed of thecomposition past the irradiating light, molecular weight of the producedpolymer can be improved (increased) and the distribution (range) ofmolecular weights in the polymer product reduced, and physical and otherproperties of the produced polymer can be improved.

1. Free radical polymerization method, comprising: providing acomposition containing a material that can be polymerized by freeradical polymerization; and polymerizing said material in saidcomposition by irradiating said composition with radiation generated bya lamp driven by a DC power supply so as to initiate and maintain saidfree radical polymerization, wherein said composition is irradiated witha substantially constant radiation, over time, substantially withoutpulsation, during said polymerizing.
 2. Method according to claim 1,wherein said lamp produces ultraviolet light, the composition beingirradiated with the ultraviolet light so as to initiate and maintain thefree radical polymerization.
 3. Method according to claim 2, wherein thelamp is an electrodeless lamp.
 4. Method according to claim 1, whereinsaid lamp is excited, to produce said radiation, by microwaves, themicrowaves being generated by a microwave source powered by said DCpower supply.
 5. Method according to claim 4, wherein said lamp is anelectrodeless lamp.
 6. Method according to claim 1, wherein said lamp isan arc lamp.
 7. Apparatus for performing free radical polymerization,comprising: support structure for holding a composition that can bepolymerized by free radical polymerizaton; a lamp for irradiating thecomposition with radiation so as to initiate and maintain the freeradical polymerization, the lamp being adapted to provide asubstantially constant radiation output, over time, substantiallywithout pulsation, for irradiating the composition when performing thefree radical polymerization; and a DC power supply for driving saidlamp.
 8. Apparatus according to claim 7, wherein said lamp is a lampgenerating ultraviolet light, whereby said radiation is ultravioletlight.
 9. Apparatus according to claim 7, wherein said lamp is an arclamp.
 10. Apparatus according to claim 7, wherein said lamp is anelectrodeless lamp and wherein said apparatus further comprises amicrowave source, the electrodeless lamp being excited by microwavesfrom said microwave source, and said microwave source being powered bysaid DC power supply.
 11. Apparatus according to claim 10, wherein saidmicrowave source is a magnetron.